In the past, in an optical transmission system, in some cases, a distributed Raman amplifier (DRA) has been used. By injecting pump light into an optical transmission path and using the stimulated Raman scattering of the optical transmission path, the DRA amplifies an optical signal propagating through the optical transmission path. Using the DRA, the input power of the optical signal input to an optical receiver becomes large, and the ratio of optical signal power to optical noise power (signal to noise ratio: SNR) becomes large. Accordingly, a transmission characteristic is improved.
Incidentally, there has been a method where, by comparing bit data, obtained by the photoelectric conversion of a received optical signal, with a threshold value controlled by automatic threshold control (ATC), the error rate of digital output data is reduced. Such a method is disclosed in, for example, Japanese Laid-open Patent Publication No. 2000-201113. In addition, there has been a device where, on the basis of a result obtained by comparing an initial threshold value with a threshold value corresponding to the maximum power of an optical noise, by comparing one of the threshold values with the input power of an optical signal during operation, the detection accuracy of the loss of light is improved. Such a device is disclosed in, for example, Japanese Laid-open Patent Publication No. 2008-236116.